[dm-devel] Shell Scripts or Arbitrary Priority Callouts?

[John A. Sullivan III]

On Wed, 2009-03-25 at 17:52 +0200, Pasi Kärkkäinen wrote:
> On Tue, Mar 24, 2009 at 11:41:00PM -0400, John A. Sullivan III wrote:
> > > > Latency seems to be our key.  If I can add only 20 micro-seconds of
> > > > latency from initiator and target each, that would be roughly 200 micro
> > > > seconds.  That would almost triple the throughput from what we are
> > > > currently seeing.
> > > > 
> > > 
> > > Indeed :) 
> > > 
> > > > Unfortunately, I'm a bit ignorant of tweaking networks on opensolaris.
> > > > I can certainly learn but am I headed in the right direction or is this
> > > > direction of investigation misguided? Thanks - John
> > > > 
> > > 
> > > Low latency is the key for good (iSCSI) SAN performance, as it directly
> > > gives you more (possible) IOPS. 
> > > 
> > > Other option is to configure software/settings so that there are multiple
> > > outstanding IO's on the fly.. then you're not limited with the latency (so much).
> > > 
> > > -- Pasi
> > <snip>
> > Ross has been of enormous help offline.  Indeed, disabling jumbo packets
> > produced an almost 50% increase in single threaded throughput.  We are
> > pretty well set although still a bit disappointed in the latency we are
> > seeing in opensolaris and have escalated to the vendor about addressing
> > it.
> > 
> 
> Ok. That's pretty big increase. Did you figure out why that happens? 
Greater latency with jumbo packets.
> 
> > The once piece which is still a mystery is why using four targets on
> > four separate interfaces striped with dmadm RAID0 does not produce an
> > aggregate of slightly less than four times the IOPS of a single target
> > on a single interface. This would not seem to be the out of order SCSI
> > command problem of multipath.  One of life's great mysteries yet to be
> > revealed.  Thanks again, all - John
> 
> Hmm.. maybe the out-of-order problem happens at the target? It gets IO
> requests to nearby offsets from 4 different sessions and there's some kind
> of locking or so going on? 
Ross pointed out a flaw in my test methodology.  By running one I/O at a
time, it was literally doing that - not one full RAID0 I/O but one disk
I/O apparently.  He said to truly test it, I would need to run as many
concurrent I/Os as there were disks in the array.  Thanks - John
> 
> Just guessing. 
> 
> -- Pasi
> 
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-- 
John A. Sullivan III
Open Source Development Corporation
+1 207-985-7880
jsullivan at opensourcedevel.com

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